I upgraded my CAT internet last week and this morning Gmail would not open.

I fiddled around and turned off both modem firewalls, it now works fine but I worry about what else will creep in.

Surprised an https: connection would be blocked. 

 

Sometimes it's a SIP/ALG filter. Try toggling it and re-enable the firewall.

Linksys has had issues in the far past where AES encrypted WiFi traffic would corrupt google services, but TKIP encrypted WiFi or direct Ethernet cabled access worked fine. 

 

And, as always, see if there are any firmware updates.

Good as gold all day then about 5pm went off like someone threw a switch.

Quite baffling.

Sounds like someone threw a switch then.